We were there for New Years Eve which I am guessing will be close to the same type of crowds. The waterpark opens at 8am and I would get there as early as possible and do it before 11am and then leave and do the other things in the resort (cub club, arcade....). I am sure there will be daily activites especially for March Break. The waterpark will be packed between noon and 5pm! A lot of people go to the waterpark and throw their towels on chairs early and then leave so if you get there after noon, you most likely won't find anywhere to sit!
Checking in could be a real dissapointment. Given they are most likely sold out, you won't get you room early at all! We were told we could not have our room until 4pm, so we walked down the hall to our room (they gave us the number) and slipped the cleaning lady $20 to clean our room next so we got it at around 2pm. You can use the waterpark after 1pm so I would plan on having a separate bag to take to the waterpark, leave your stuff in the car, and go and have fun until 4pm.
If you get there before 1pm, they will send you away. They wouldn't even let us do the paperwork while there was noone in line. She said "check-in is after 1pm". I tried again at noon with another employee and she did check us in at around 12:30 so we could get the wristbands and use the waterpark.